From mboxrd@z Thu Jan 1 00:00:00 1970 From: mturquette@linaro.org (Michael Turquette) Date: Thu, 30 Jul 2015 15:47:20 -0700 Subject: [PATCH v7 3/5] clk: Supply the critical clock {init, enable, disable} framework In-Reply-To: <20150730095014.GD14642@x1> References: <1437570255-21049-1-git-send-email-lee.jones@linaro.org> <1437570255-21049-4-git-send-email-lee.jones@linaro.org> <20150727072549.GP2564@lukather> <20150727085338.GW3436@x1> <20150728114022.GW2564@lukather> <20150728130055.GV14943@x1> <20150730011932.642.85168@quantum> <20150730095014.GD14642@x1> Message-ID: <20150730224720.23791.6722@quantum> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Quoting Lee Jones (2015-07-30 02:50:14) > On Wed, 29 Jul 2015, Michael Turquette wrote: > > Quoting Lee Jones (2015-07-28 06:00:55) > > > On Tue, 28 Jul 2015, Maxime Ripard wrote: > > > > > > > On Mon, Jul 27, 2015 at 09:53:38AM +0100, Lee Jones wrote: > > > > > On Mon, 27 Jul 2015, Maxime Ripard wrote: > > > > > > > > > > > On Wed, Jul 22, 2015 at 02:04:13PM +0100, Lee Jones wrote: > > > > > > > These new API calls will firstly provide a mechanisms to tag a clock as > > > > > > > critical and secondly allow any knowledgeable driver to (un)gate clocks, > > > > > > > even if they are marked as critical. > > > > > > > > > > > > > > Suggested-by: Maxime Ripard > > > > > > > Signed-off-by: Lee Jones > > > > > > > --- > > > > > > > drivers/clk/clk.c | 45 ++++++++++++++++++++++++++++++++++++++++++++ > > > > > > > include/linux/clk-provider.h | 2 ++ > > > > > > > include/linux/clk.h | 30 +++++++++++++++++++++++++++++ > > > > > > > 3 files changed, 77 insertions(+) > > > > > > > > > > > > > > diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c > > > > > > > index 61c3fc5..486b1da 100644 > > > > > > > --- a/drivers/clk/clk.c > > > > > > > +++ b/drivers/clk/clk.c > > > > > > > @@ -46,6 +46,21 @@ static struct clk_core *clk_core_lookup(const char *name); > > > > > > > > > > > > > > /*** private data structures ***/ > > > > > > > > > > > > > > +/** > > > > > > > + * struct critical - Provides 'play' over critical clocks. A clock can be > > > > > > > + * marked as critical, meaning that it should not be > > > > > > > + * disabled. However, if a driver which is aware of the > > > > > > > + * critical behaviour wants to control it, it can do so > > > > > > > + * using clk_enable_critical() and clk_disable_critical(). > > > > > > > + * > > > > > > > + * @enabled Is clock critical? Once set, doesn't change > > > > > > > + * @leave_on Self explanatory. Can be disabled by knowledgeable drivers > > > > > > > + */ > > > > > > > +struct critical { > > > > > > > + bool enabled; > > > > > > > + bool leave_on; > > > > > > > +}; > > > > > > > + > > > > > > > struct clk_core { > > > > > > > const char *name; > > > > > > > const struct clk_ops *ops; > > > > > > > @@ -75,6 +90,7 @@ struct clk_core { > > > > > > > struct dentry *dentry; > > > > > > > #endif > > > > > > > struct kref ref; > > > > > > > + struct critical critical; > > > > > > > }; > > > > > > > > > > > > > > struct clk { > > > > > > > @@ -995,6 +1011,10 @@ static void clk_core_disable(struct clk_core *clk) > > > > > > > if (WARN_ON(clk->enable_count == 0)) > > > > > > > return; > > > > > > > > > > > > > > + /* Refuse to turn off a critical clock */ > > > > > > > + if (clk->enable_count == 1 && clk->critical.leave_on) > > > > > > > + return; > > > > > > > + > > > > > > > > > > > > I think it should be handled by a separate counting. Otherwise, if you > > > > > > have two users that marked the clock as critical, and then one of them > > > > > > disable it... > > > > > > > > > > > > > if (--clk->enable_count > 0) > > > > > > > return; > > > > > > > > > > > > > > @@ -1037,6 +1057,13 @@ void clk_disable(struct clk *clk) > > > > > > > } > > > > > > > EXPORT_SYMBOL_GPL(clk_disable); > > > > > > > > > > > > > > +void clk_disable_critical(struct clk *clk) > > > > > > > +{ > > > > > > > + clk->core->critical.leave_on = false; > > > > > > > > > > > > .. you just lost the fact that it was critical in the first place. > > > > > > > > > > I thought about both of these points, which is why I came up with this > > > > > strategy. > > > > > > > > > > Any device which uses the *_critical() API should a) have knowledge of > > > > > what happens when a particular critical clock is gated and b) have > > > > > thought about the consequences. > > > > > > > > Indeed. > > > > > > > > > I don't think we can use reference counting, because we'd need as > > > > > many critical clock owners as there are critical clocks. > > > > > > > > Which we can have if we replace the call to clk_prepare_enable you add > > > > in your fourth patch in __set_critical_clocks. > > > > > > What should it be replaced with? > > > > > > > > Cast your mind back to the reasons for this critical clock API. One > > > > > of the most important intentions of this API is the requirement > > > > > mitigation for each of the critical clocks to have an owner > > > > > (driver). > > > > > > > > > > With regards to your second point, that's what 'critical.enabled' > > > > > is for. Take a look at clk_enable_critical(). > > > > > > > > I don't think this addresses the issue, if you just throw more > > > > customers at it, the issue remain with your implementation. > > > > > > > > If you have three customers that used the critical API, and if on of > > > > these calls clk_disable_critical, you're losing leave_on. > > > > > > That's the idea. See my point above, the one you replied "Indeed" > > > to. So when a driver uses clk_disable_critical() it's saying, "I know > > > why this clock is a critical clock, and I know that nothing terrible > > > will happen if I disable it, as I have that covered". So then if it's > > > not the last user to call clk_disable(), the last one out the door > > > will be allowed to finally gate the clock, regardless whether it's > > > critical aware or not. > > > > > > Then, when we come to enable the clock again, the critical aware user > > > then re-marks the clock as leave_on, so not critical un-aware user can > > > take the final reference and disable the clock. > > > > > > > Which means that if there's one of the two users left that calls > > > > clk_disable on it, the clock will actually be disabled, which is > > > > clearly not what we want to do, as we have still a user that want the > > > > clock to be enabled. > > > > > > That's not what happens (at least it shouldn't if I've coded it up > > > right). The API _still_ requires all of the users to give-up their > > > reference. > > > > > > > It would be much more robust to have another count for the critical > > > > stuff, initialised to one by the __set_critical_clocks function. > > > > > > If I understand you correctly, we already have a count. We use the > > > original reference count. No need for one of our own. > > > > > > Using your RAM Clock (Clock 4) as an example > > > -------------------------------------------- > > > > > > Early start-up: > > > Clock 4 is marked as critical and a reference is taken (ref == 1) > > > > > > Driver probe: > > > SPI enables Clock 4 (ref == 2) > > > I2C enables Clock 4 (ref == 3) > > > > > > Suspend (without RAM driver's permission): > > > SPI disables Clock 4 (ref == 2) > > > I2C disables Clock 4 (ref == 1) > > > /* > > > * Clock won't be gated because: > > > * .leave_on is True - can't dec final reference > > > > I am clearly missing the point. The clock won't be gated because the > > enable_count is still 1! What does .leave_on do here? > > The point of _this_ (the extended) part of the API is so that the > clock _can_ be turned off. Without the possibility to disable > .leave_on and the logic with accompanies it (i.e. > clk_disable_critical()) the clock will _never_ be gated. > > > > */ > > > > > > Suspend (with RAM driver's permission): > > > /* Order is unimportant */ > > > SPI disables Clock 4 (ref == 2) > > > RAM disables Clock 4 (ref == 1) /* Won't turn off here (ref > 0) > > > I2C disables Clock 4 (ref == 0) /* (.leave_on == False) last ref can be taken */ > > > /* > > > * Clock will be gated because: > > > * .leave_on is False, so (ref == 0) > > > > Again, .leave_on does nothing new here. We gate the clock because the > > reference count is 0. > > It's the fact that .leave_on has been disabled in > clk_disable_critical() that allows the final reference to be taken. > > > > */ > > > > > > Resume: > > > /* Order is unimportant */ > > > SPI enables Clock 4 (ref == 1) > > > RAM enables Clock 4 and re-enables .leave_on (ref == 2) > > > I2C enables Clock 4 (ref == 3) > > > > Same again. As soon as RAM calls clk_enable_critical the ref count goes > > up. .leave_on does nothing as far as I can tell. The all works because > > of the reference counting, which already exists before this patch > > series. > > So fundamentally you're right in what you say. All you really need to > disable a critical clock is write a knowledgeable driver, which is > intentionally unbalanced i.e. just calls clk_disable(). All this OK, the line above is helpful. What you really want is a formalized hand-off mechanism, whereby the clock is enabled at registration-time and it cannot be turned off until the right driver claims it and decides turning it off is OK (with a priori knowledge that the clock is already enabled). Note that I don't think this implementation can really work in the near future. Today we do not catch unbalanced calls to clk_enable and clk_disable, but I have a patch that catches this and WARNs loudly in my private tree. More on that in the next stanza. What I don't understand is if there is ever a case for a clock consumer driver to ever call clk_enable_critical... I do not think there is. What you're trying to protect against is having the clock disabled BEFORE that "knowledgeable driver" has a chance to enable it. Let me know if I've got that right. The only user of this function in your series is the clk-conf.c stuff, which matches my summary above. > extended API really does is makes the process more official and > ensures that an unintentionally unbalanced driver doesn't bugger up > the running platform. We could also add a new WARN() to say that said > driver is unbalanced, as it just tried to turn off a critical clock. As I mentioned up above I am working on this right now. Our per-user struct clk stuff makes it trivial to track prepare_count and enable_count values on a per-user basis. Consequently a naive approach that simply calls clk_disable an extra time will not work once this code is merged. This is because the struct clk used in clk-conf.c and in your knowledgeable driver will be two distinct instances. Regards, Mike > > What do you think is best? > > -- > Lee Jones > Linaro STMicroelectronics Landing Team Lead > Linaro.org ? Open source software for ARM SoCs > Follow Linaro: Facebook | Twitter | Blog